This invention relates to the preparation of 3,3,3-trifluoropropanal, CF.sub.3 CH.sub.2 CHO, and its oxime. More particularly, it relates to an improved process for forming 3,3,3-trifluoropropanal and 3,3,3-trifluoropropanal oxime.
Fluorinated compounds, particularly trifluoromethylated compounds, are known for enhancing the effectiveness of agricultural products and pharmaceuticals. Examples of extensively used trifluoromethyl compounds include trifluoroethanol, trifluoroacetic acid, trifluoroacetaldehyde, and trifluoroacetyl chloride. Higher molecular weight homologs are generally not commercially available in bulk quantities since preparations which are convenient, economical, and environmentally acceptable, have yet to be developed.
Several methods have been developed for the preparation of trifluoropropanal. However, reduction of the corresponding acid chloride or oxidation of the corresponding alcohol suffers from the lack of availability of the precursor, and variable yields. Specific problems associated with these approaches can be found in U.S. Pat. No. 5,777,184. Better methods use CF.sub.3 CH.dbd.CHOR, where R is usually methyl or ethyl, as a key intermediate. These enol ethers can be made by the addition of methanol or ethanol to trifluoropropyne. For example, the addition of NaOMe to trifluoropropyne gives Z--CF.sub.3 CH.dbd.CHOMe (R. N. Hazeldine, J. Chem. Soc., 1952, 3490). However, trifluoropropyne is not readily available. The methyl enol ether can be prepared by reacting 2-bromo-3,3,3-trifluoropropene with NaOH in aqueous methanol. (F. Hong and C-M. Hu, J. Chem. Soc., Chem. Comm. 1996, 57).
Furthermore, 1-chloro-3,3,3-trifluoropropene (CF.sub.3 CH.dbd.CHCl) has also been used as a starting material for the preparation of trifluoropropanal. U.S. Pat. No. 5,777,184 outlines a method where the chloride is converted into the corresponding acetate in a palladium-catalyzed process, which is subsequently hydrolyzed to the aldehyde. The preferred method for the preparation of CF.sub.3 CH.dbd.CHCl is given in Example 1 in U.S. Pat. No. 5,777,184 which is incorporated herein by reference. This process suffers from relatively few catalyst turnovers before deactivation occurs. In another process, described in U.S. Pat. No. 2,739, 987, the chloride is converted into E--CF.sub.3 CH.dbd.CHOMe with KOH in methanol. This process not only gives a different isomer of CF.sub.3 CH.dbd.CHOMe than that obtained from either trifluoropropyne or CF.sub.3 CBr.dbd.CH.sub.2, but the yield of the enol ether is substantially lower.
Even if CF.sub.3 CH.dbd.CHOMe were available, its conversion to trifluoropropanal is still problematic. The enol ether has been known to behave very unusually in that reduction using freshly prepared Raney-nickel gives trifluoropropanal (R. N. Hazeldine, J Chem. Soc., 1952, 3490). Although the yield was high, a large weight of catalyst is used compared to the organic starting material. Furthermore, the aldehyde is isolated only as a derivative. Aqueous HI can be used to convert CF.sub.3 CH.dbd.CHOMe to the aldehyde (R. P. Ruh., U.S. Pat. No. 2,739,987, 1965). Although the yield is good, aqueous HI is expensive. Pazenok et al. (J. Org. Chem. USSR, Engl. Transl. 25 (1989) 1238) hydrolyzed the enol ether with dilute mineral acid (10% HCl), a standard method for this transformation. However, they found the stoichiometry to be
2CF.sub.3 CH.dbd.CHOMe.fwdarw.CF.sub.3 CH.sub.2 CHO+CF.sub.3 CH.sub.2 CH(OMe).sub.2 indicating that the acetal is stable to acid hydrolysis (a standard method for converting acetals to aldehydes). The yield of aldehyde, based on this stoichiometry was only 44%, while the yield of acetal was 76%. Consequently, the prior art methods for making the aldehyde from CF.sub.3 CH.dbd.CHCl have a very poor overall yield.
It would be desirable to provide a process for making 3,3,3-trifluoropropanal which is easier, less expensive, and has a higher yield that other known methods. This invention provides an improved process for producing 3,3,3-trifluoropropanal, as well as a process for making a previously unknown compound, 3,3,3-trifluoropropanal oxime.